THE PLEISTOCENE OR GLACIAL PERIOD 911 



the sea. They reach altitudes of 200 or 300 feet in California 1 

 and Oregon, and perhaps' even higher. The submergence indicated 

 by the position of these beds must have given origin to considerable 

 bays in the lower courses of the Columbia and Willamette valleys. 

 By far the larger part of the marine Quaternary deposits of the 

 coasts of the continent are still beneath the sea. 



Igneous rocks. The Quaternary eruptions of North America 

 have not been clearly separated from those of the late Tertiary, 

 but there are some igneous rocks which are Quaternary, some of 

 them even late Quaternary. Mount Shasta shows several post- 

 glacial lava-flows, 2 and there are small cinder cones on alluvial 

 cones at the east base of the Sierras in southeastern California. In 

 southern California (Mohave desert) and northern Arizona (vicinity 

 of Flagstaff) there are cinder cones, and lava- flows of limited extent 

 which are so slightly touched by erosion that there can be little 

 doubt that they date from a time long subsequent to the beginning 

 of the Quaternary period. Judged by the same criteria, there are 

 lava-flows and cinder cones of Quaternary age in New Mexico, 

 Colorado, Utah, Nevada, Oregon, (p. 382), Idaho, 3 Washingf on, 4 

 and at various points in the Sierras. 5 On many of them vegetation 

 has hardly begun to gain a foothold. Gilbert estimates that of 

 250 lava-fields observed in these states, 15% are of Pleistocene age, 

 and of 350 volcanic cones in the same states, 60% are considered 

 to be Pleistocene. 6 Volcanic ash is interbedded with loess at vari- 

 ous points in eastern Washington and Oregon, 7 and overlies glacial 

 moraines in some parts of Alaska. 



CHANGES OF LEVEL DURING THE PLEISTOCENE 



The very considerable changes of level which marked the closing 

 stages of the Pliocene have been mentioned, and many of them 



1 Ashley, Jour. GeoL, Vol. Ill, pp. 446-^50. 



2 Diller, Physiography of the United States, pp. 245 et seq. 



3 Nampa, Ida., folio, U. S. Geol. Surv.; also Russell, Bull. 217, U. S. Geol. 

 Surv. 



4 Tacoma and Ellensburg, Wash., folios, U. S. Geol. Surv. 



5 See Bidwell Bar, Colfax, Downieville, Lassen Peak, Pyramid Peak and 

 Truckee, CaL, folios, U. S. Geol. Surv. 



6 Mono. I, U. S. Geol. Surv., pp. 323-337. 



7 Jour. Geol., Vol. IX, p. 730. 



